RESULTS AND DISCUSSION
Results obtained by application of the modified method to nine inorganic nitrates are given in Table I. Typical values obtained by the unmodified Dumas method are included for comparison. A variety of organic materials could be substituted for the anthracene in this application, but judging from the rather low results obtained with benzoic acid and sucrose, the use of a hydrocarbon is indicated. Anthracene is readily available in pure and easily handled crystalline form. The results using anthracene are considered sufficiently accurate for many purposes. Although the author’s experience is limited to the materials listed in Table I, the procedure could be ap-
plied generally to inorganic nitrates. Using the Coleman nitrogen analyzer, 8 minutes are required for an analysis. The Dumas nitrogen analysis of highly carbonaceous materials often results in the production of methane and therefore leads to high results (6, 6). When as much as 15 mg. of anthracene were run through the Coleman nitrogen analyzer, there was no apparent increase in the normal blank. Therefore, the formation of methane, as a result of the addition of 7 to 10 mg. of anthracene to the nitrate samples in Table I, was considered negligible. However, if more than 1Fj mg. of anthracene are used, or if another hydrocarbon is substituted for anthracene, then the blank should be checked for possible formation of methane.
ACKNOWLEDGMENT
The author expresses his appreciation to hnna C. Darling who performed the analyses listed in Table I. LITERATURE CITED
(1) Gustin, G. W., Microchem. J. 4 , 43-54 (1960). (2) Klimova, T’. A., Dubinina, I. F., I z v . Akad. ib-azck SSSR, Otd. Khim. iYauk 1958, (2), p. 129-32. (3) Kolthoff, I. M., Stenger, 1‘. A., ‘‘1olumetric Analysis 11, Titration Methods,” p. 172, Interscience, New York, 1947. (4) “Scott’s Standard Methods of Chemical Analysis,” p. 640, N. H. Furman, ed., Van Sostrand, Kew York, 1939. ( 5 ) Stewart, B. A., Porter, L. K., Beard, W. E., ANAL.CHEM.,9, 1331-2 (1963). (6) Steyermark, A., Quantitative Organic Microanalysis,” 2nd ed., p. 151,
Arademic Press, New York and London, 1961.
A Convenient Cell Design for Studies Employing Shielded Solid Electrodes Harry
B. Mark,
Jr., Department of Chemistry, The University of Michigan, Ann Arbor, Mich.
in chronopotentiometric Itimesstudies an infinite range of transition is desired. However, error is DEALLY,
introduced in the measurement of both short and long transition times ( 2 ) . The error in measuring short transition times can be empirically corrected (2). However, the effects of nonlinearity of diffusion and also convection which result from differences in the relative density of the electrode reactant and product can be minimized only by employing a shielded electrode of proper orientation (2). Such electrodes give a wide range of reliable 7 values
(1, 2, 4). In some cases the shielded electrode is, however, difficult to work with in a mechanical sense. The cell design given here eliminates these difficulties. Primarily, in the downward horizontal orientation [this position is employed to reduce convection when the relative density of the solution produced on electrolysis is less than the original solution ( 2 ) ] of the shielded electrode, the cavity (see Figure 1) formed by shield is difficult to fill with sample solution because of the air bubble trapped in it on immersion. Bard ( 2 )
and Johnson (4) employed a syringe which had a curved tip that was inserted in the cavity to draw the air bubble out. I t is difficult, however, to see the bubble in the cavity--let alone to remove all of it in this manner-and this is virtually an impossible task if the sample solution is highly colored. Furthermore, on deaeration with Nz following removal of the bubble, Nz bubbles generally collect in the cavity, necessitating another removal operation before a measurement can be made. To circumvent this problem, a Teflon stopcock assembly, S , was sealed into
ADJUSTABLE
AUX. ELECTRODE
TEFLON CELL COVER REF.(or AUX.) ELECT. COMPART.
B
STIRRING BAR
Figure 1. assembly
958
Teflon-shielded electrode
END VIEW Figure 2.
ANALYTICAL CHEMISTRY
SIDE VIEW
Variable orientation-shielded electrode cell
the side of a standard H cell and the shielded electrode wm mounted, as shown in Figure 2, on a bent (90’) glass tube, T, which was inserted through a hole drilled lengthwise in the Teflon stopper. Mounted in this manner, the shielded electrode can be rotated through 180’ by turning the Teflon stopcock and needs only to be turned in an upward orientation to expel any air bubble. It is then ro1;ated into a downward orientation for measurements requiring such an orientation. One added advantage of this cell is that any desired orientation for a measurement is readily available without having to make a separate electrode for each position. The bulge design in the wall of the sample compa,rtment of the H cell, through which the electrode rotates freely, was employelj to reduce the total volume of sample required for measurement. A rubber band, R, was crossed over the stopcock arms and attached to two small glass hooks, h, on the side of the cell to maintain pressure on the stopcock. The stopcock could be rotated freely without removing the
rubber band. The holes in the Teflon stopcock and in the base of the shielded electrode assembly for the tube, T,were drilled in such a way as to give a tight fit. No further sealing at these joints was necessary, as no leaking into or around the tube, T , was observed even over a long period of time. The Teflon-shielded electrode assembly (see Figure 1) is essentially identical, except in size, to that designed by Johnson (4). This electrode assembly was used instead of a platinum disk sealed in soft glass (2) because it was convenient a t times to be able to remove the Pt disk for cleaning and polishing (4, especially when studying film deposition and dissolution and, also, some organic electrode reactions. The cell was designed so that the shield cap, C, could be unscrewed without removing the whole assembly from T. Electrical contact with the Pt disk was maintained through a steel coil spring (stretched to 21/2 times its original length) of approximately the same diameter as the inside diameter of the tube, T. The compression of this spring on
insertion into T resulted in a slight pressure of the end of the spring on the back of the Pt electrode which ensured a low resistance contact. The bevel, B, of the inside of the Teflon electrode shield cap, C, ensures a watertight seal between the Pt electrode and the bottom edge of the shield when screwed down tightly. This was tested by dipping the electrode in Fe+3 solution, washing and transferring the electrode to a 1.0 HCIOl ’ solution, and observing the resulting chronopotentiogram ( 3 ) . N o Fe+3 reduction wave was observed, indicating that there were no small cracks for Fe+3 to diffuse into ( 3 ) . LITERATURE CITED
(1) Adams, R. N., Symposium on Elec-
trode Reaction Mechanisms, Division of Analytical Chemistry, 145th hleeting, ACS, New York, September 1963. (2) Bard, A. J., ANAL. CHEM. 33, 11
(1961); 35, 340 (1963). (3) Christensen, C. R., Anson, F. C., Ibid., p. 340. (4)Johnson, J. D., P1i.D. dissertation, University of North Carolina, Chapel Hill, N. C., 1962.
Automatic Mass Scanner for a Time-of-Flight Mass Spectrometer M. V. McDowell, R. S. Olfky, and F. E. Saalfeld, U. S. Naval Research Laboratory, Washington, D. C. 20390 a mass R group of masses by a mass spectrometer has many applications, for EPETITIOUS SCANNING O f
Or
example, in the moni1,oring of products from an effusion cell or in following the progress of a chemical reaction. Peak monitors employing a scanning circuit similar to that shown in Figure 1 exist for magnetic deflection-type mass spectrometers (for exitmple, the auto-
matic peak selector, type 21-065, made for the Consolidated Electrodynamics Corp. 21-620 mass spectrometer, allows up to six preselected peaks to be monitored, but does not have the ability to scan the entire mass spectrum), but there are no commercial automatic mass scanners available for the Bendix time-of-flight (T-0-F) mass spectrometer.
The scanning system described herein can be programmed so the circuit on the analog output chassis of the mass spectrometer and the recorder (Leeds and Northrup Speed-0-Matic G) are turned on automatically and remain on for any desired length of scan. This enables one peak or a series of consecutive peaks to be observed. After the scan has been completed, the scanner and the recorder are turned off and remain so for a preset length of time, after which the scan is automatically
I -+t 69 MIDDLE DISK2 DEC.
VDC
i3-P DISK 3
LEEDS 8 NORTHRUP RECORDER CONTROL
-TO
MOTOR(I-RPM) -DRIVING CAMS
POTTER 8 BRUMFIELD KRP14A 3PDT RELAY
VDC
*
SCHEMATIC IDENTIFICATIONS ARE FROM INSTRUCTION MANUAL &NALOG OUTPUT SYSTEM ELECTRONIC CHASSIS MODEL 30 MONITOR MODEL 311 CONTROLLER MODEL 311 SCANNER ACCESSORY FOR BENDIX T-0-F MASS SPECTROMETER
Figure 1 .
Scanning circuit
kl
I -
7 DPST
Figure 2.
Timing circuit VOL. 36, NO. 4, APRIL 1964
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0
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